The present invention relates to a surface acoustic wave device which is applicable to high frequencies.
In general, an interdigital transducer (IDT) is employed as an electromechanical transducer of a surface acoustic wave device, and the width W of the finger of a comb-shaped electrode in the IDT is proportionate to the wavelength .lambda. (generally the width W is 1/4 or 1/8 of the wavelength .lambda.), that is, inversely proportional to the frequency f, and thus the width W is narrowed in a high frequency device, wherefore formation of the IDT by, e.g., photoetching is made difficult.
However, since it is well known in the art that the relation between the frequency f, the wavelength .lambda. and the SAW velocity .nu. is represented by the equation .lambda.=.nu./f, the wavelength .lambda. is increased by raising the SAW velocity .nu., wherefore formation of the IDT is simplified even if the device is applied to high frequencies.
Further, elecromechanical conversion efficiency of the IDT depends on the square value k.sup.2 of an electromechanical coupling coefficient k of a substrate employed therein, and the conversion efficiency is improved as the value k is increased.
Thus, there is required a substrate of high SAW velocity having a high coupling coefficient as the substrate to be employed in a high frequency surface acoustic wave device.
With respect to a substrate generally employed in a conventional surface acoustic wave device, lithium niobate (LiNbO.sub.3) is selected as the material applicable to high frequencies with the SAW velocity .nu. being about 4000 m/s and the square value k.sup.2 of the coupling coefficient k being equal to 5.6%. On the other hand, there has been proposed a substrate member utilizing so-called Sezawa wave or higher mode by forming zinc oxide (ZnO) film on a sapphire substrate or an alumina ceramic substrate. In such structure, the SAW velocity .nu. is 5500 to 6000 m/s, which is over 1.4 times as large as that of the LiNbO.sub.3 substrate though the value k.sup.2 of about 5% is similar to that of the LiNbO.sub.3 substrate, and in view of the high SAW velocity value .nu., ZnO on the sapphire substrate is more suitable as the material for the substrate applicable to a high frequency device. However, sapphire is considerably expensive in comparison with LiNbO.sub.3. Further, with respect to the alumina ceramic substrate, it is difficult to obtain a mirror face suitable for formation of a fine IDT pattern while the propagation loss is increased in a high frequency range since the same is a sintered substance having large grain sizes, wherefore it is difficult to apply the alumina ceramic substrate to a high frequency device.